Crystalline microporous molecular sieves, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, the crystalline microporous molecular sieves have been used as adsorbents and catalyst carriers for various types of hydrocarbon conversion processes, and other applications. These molecular sieves are ordered, porous, crystalline materials having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. The dimensions of these channels or pores are such as to allow for adsorption of molecules with certain dimensions while rejecting those of large dimensions. The interstitial spaces or channels formed by the crystalline network enable molecular sieves such as crystalline silicates, crystalline aluminosilicates crystalline silicoalumino phosphates, and crystalline aluminophosphates, to be used as molecular sieves in separation processes and catalysts and catalyst supports in a wide variety of hydrocarbon conversion processes.
Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term "zeolites" includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, titanium oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms "zeolite", "zeolites" and "zeolite material", as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term "aluminosilicate zeolite", as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
Zeolites such as ZSM-5, that have been combined with a Group VIII metal have been used in the past as catalysts for hydrocarbon conversion. For example, U.S. Pat. No. 3,856,872 discloses a zeolite preferably containing a binder such as aluminia that has been loaded with platinum by impregnation or ion exchange. A problem associated with zeolite catalysts that have been loaded with metals by impregnation or ion exchange is that the metal may not be well dispersed. If the metal is not well dispersed, selectivity, activity and/or activity maintenance of the zeolite catalyst can be adversely effected.
U.S. Pat. No. 4,312,790 discloses another method of loading platinum on an alumina bound zeolite. The method involves adding the noble metal to the zeolite after crystallization of the zeolite, but before calcination. Catalysts prepared by this method have not been commercially useful because, as reported in U.S. Pat. No. 4,683,214, the use of the method has resulted in catalysts with poor platinum dispersion and large platinum crystallites.
Synthetic zeolites are normally prepared by the crystallization of zeolites from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although the zeolite powder has good adsorptive properties, its practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using the powder in commercial processes, the zeolite crystals are usually bound.
The zeolite powder is typically bound by forming a zeolite aggregate such as a pill, sphere, or extrudate. The extrudate is usually formed by extruding the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. Examples of binder materials include amorphous materials such alumina, silica, titania, and various types of clays. It is generally necessary that the zeolite be resistant to mechanical attrition, that is, the formation of fines which are small particles, e.g., particles having a size of less than 20 microns.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when such a bound zeolite is used in a catalytic conversion process, the performance of the zeolite catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the binder. For instance, since the binder is typically present in an amount of up to about 50 wt. % of zeolite, the binder dilutes the adsorption properties of the zeolite aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite when used in xylene isomerization. Furthermore, when the bound zeolite is used in catalytic conversion processes, the binder may affect the chemical reactions that are taking place within the zeolite and also may itself catalyze undesirable reactions which can result in the formation of undesirable products.
In certain hydrocarbon conversion processes involving dehydrogenation and dehydrocyclization reactions, it is desirable that the zeolite catalyst used in the process be effective for metal-catalyzed reactions, e.g., conversion of paraffins to aromatic products. In order for the catalyst to be effective for metal catalyzed reactions, a catalytically active metal is usually included in the catalyst. The catalytically active metal should be uniformly dispersed. If the metal is not uniformly dispersed, the activity, selectivity, and/or activity maintenance of the catalyst can be adversely effected.
Accordingly, it would be desirable to produce zeolite catalysts which have uniformly dispersed hydrogenation/dehydrogenation metals and do not contain substantial amounts of non-zeolitic binder.